(a) Field of the Invention
The invention relates to superconducting devices, corresponding technologies and application fields, and more specifically to a novel generator and detector of sub-millimeter electromagnetic radiation, and its multiple applications.
(b) Description of the Related Art
1. Arrays of Artificial Junctions
The first realization of potential usefulness of Josephson junctions as tunable microwave sources and detectors can be traced back to the earliest works of B. Josephson and S. Shapiro. It was also understood very early that a single Josephson junction emits with too little power and too broad linewidth to be useful as a practical microwave source. These deficiencies can be removed by using arrays of Josephson junctions [Jain et al. 1984; Bindslev Hansen and Lindelof 1984; Lukens 1990]. If the coupling between the junctions is strong enough, phase locking may occur between them; in this cases, the array emits coherent radiation [Lukens 1990; Konopka 1994]. Possible coupling mechanisms and coupling strengths have been analyzed in detail [Jain et al. 1984; Lukens 1990]. It has been understood that the linewidth of the electromagnetic radiation emitted from an array of Josephson junctions decreases as the number of junctions within the array is increased, and can become very narrow in large arrays [Lukens 1990; Wiesenfeld et al. 1994; Konopka 1994].
Power of the emitted radiation also increases with the number of junctions in the array, and in large arrays it can become large enough (Pxe2x89xa71 mW) for many practical applications [Bindslev Hansen and Lindelof 1984; Jain et al. 1984; Konopka et al. 1994; Wiesenfeld et al. 1994]. It is important here that a good impedance matching is achieved with the load, because in the opposite case most of the radiation is reflected back and dissipated within the device itself [Jain et al. 1984; Bindslev Hansen and Lindelof 1984; Konopka 1994].
Another concern are various junction parasitics; for example, junction capacitances are a source of power reduction at higher frequencies [Lukens 1990; Wiesenfeld et al. 1994]. This favors small-area Josephson junctions. Another argument pointing to the same conclusion is increased noise and linewidth broadening in large-area junctions [Kunkel and Siegel 1994; Konopka 1994]. Technically, for wxe2x89xa74xcexj, where w is the junction width and xcexj is the Josephson penetration depth, the current flow becomes inhomogeneous [Kunkel and Siegel 1994]. It has been understood also that to achieve complete phase locking in an array of coupled Josephson junctions, it is necessary that all the junctions within the array have similar critical currents (Ic); in general, uniformity of xc2x15% or better is required for linear arrays [see e.g. Konopka 1994].
It is possible to relax somewhat the above stringent requirements by using a distributed arrays of equidistant Josephson junctions (see FIG. 8), provided that the operating frequency is adjusted in such a way to match the spacing between the junctions with the wavelength of the emitted electromagnetic radiation [Lukens 1990; Han et al. 1994]. This obviously reduces tunability in frequency, while the power of the emitted radiation can be increased significantly.
There have been numerous experimental studies of Josephson junction arrays, and some remarkable results have been achieved. Most of these were based on conventional (low-Tc) superconductors, e.g. using Nb/Alxe2x80x94AlOx/Nb, trilayer junctions. Complete phase locking has been demonstrated in a linear array of 100 such junctions [Han et al. 1993]. In some cases, a broad-band antenna (for example, a bow-tie antenna, or a two-arm logarithmic spiral antenna), was integrated on the chip, and off-chip radiation was detected and measured. In other cases, another Josephson junction was integrated on-chip and coupled via a transmission line to the array. Some of the best results include the following ones. Emission of P=50 xcexcW at xcexd=400-500 GHz was observed from a distributed array of 500 Josephson junctions [Han et al. 1994]. In another circuit design (10xc3x9710 array), radiation was generated with a linewidth as small as xcex94xcexd=10 kHz, tunable over a broad range, xcexd=53-230 GHz [Booi and Benz 1994].
With the discovery of high-temperature superconductivity in Laxe2x80x94Baxe2x80x94Cuxe2x80x94O by G. Bednorz and K. A. Mxc3xcller in 1986, and subsequent improvements of the critical temperature in related cuprate compounds up to Tc greater than 160 K, great expectations have arosen for superconductive electronics, operational at liquid nitrogen temperature and even above it. Indeed, Josephson junctions have been fabricated since 1987 in dozens of laboratories worldwide, by a variety of techniques. Emission due to ac Josephson currents are artificial high Tc Josephson junctions was measured and analyzed [Kunkel and Siegel 1994]. In the same study, phase locking of two step-edge junctions was demonstrated over a broad frequency range of xcexd=80-500 GHz. In larger arrays, only partial (up to 4 junctions) and rather unstable phase locking was observed [Konopka 1994]. This was understood to originate from a generically large non-uniformity of such step-edge high-Tc Josephson junctions, where critical current variations of xc2x150% are typical [Konopka 1990]. In another experiment five and ten-junction arrays, one next to the other, were fabricated using step-edge HTS junctions [Kunkel and Siegel 1994], again with only partial phase-locking and very small output power.
Artificial high-Tc Josephson junctions and stacks are prerequisite in one embodiment of the present invention (see section V). They have indeed been fabricated successfully already [Bozovic et al. 1994, Bozovic and Eckstein 1995, 1996a,b; Eckstein et al. 1992, 1995, Ono et al. 1995] using atomic-layer-bylayer Molecular beam epitaxy (ALL-MBE). A variety of barrier layers have been explored, including Bi2Sr2CuO6 [Bozovic and Eckstein 1996b], Bi2Sr2DyxCa1xe2x88x92xCu2O8 [Bozovic and Eckstein 1996, 1995], Bi2Sr2DyxCa1xe2x88x92xCu8O20 and BiSr2DyxCa1xe2x88x92xCu8O19 [Bozovic and Eckstein 1996, Eckstein et al. 1995], etc. High-resolution cross-sectional electron microscopy has shown virtually atomically perfect interfaces between the barriers and the superconducting electrodes [Bozovic et al. 1994b]. These multilayers were lithographically processed into mesa structures for vertical transport devices [Eckstein et al. 1992, Bozovic and Eckstein 1996b]. Both proximity-effect (SNS) junctions [Bozovic and Eckstein 1996b, 1995] and tunnel (SIS) junctions [Bozovic and Eckstein 1996a, Bozovic et al. 1994] have been fabricated in this way. They have shown remarkably high characteristic voltages, up to IcRN=10 mV (which corresponds to xcexd≈2.5 THz) and uniformity of better than xc2x15% [Bozovic and Eckstein 1996a]. It was further demonstrated that the barrier properties such as its normal state resistance RN and critical current Ic can be engineered over a very broad range (four orders of magnitude) by varying the doping level within the barrier, e.g., by varying x in the barrier layer consisting of Bi2Sr2DyxCa1xe2x88x92xCu8O20 [Bozovic and Eckstein 1996a,b, 1995, 1994a; Eckstein et al. 1992]. Finally, some short vertical stacks of such Josephson junctions have already been fabricated and they showed perfect phase locking [Bozovic and Eckstein 1996b, 1994a; Eckstein et al. 1995; Ono et al. 1995]. In conclusion, every critical technological step related to fabrication of artificial trilayer Josephson junctions, and their vertical stacks, which we assumed to be feasible in Section V (iv). below, has already been successfully demonstrated and reduced to practice.
In many of the papers mentioned here, speculative statements were made about promising future applications of arrays of Josephson junctions. For example, applications are predicted as generators and detectors of GHz and THz radiation [Jain et al. 1984], and even more specifically in radio-astronomy and radio-spectroscopy of heavy molecules [Konopka 1994], in voltage standards [Ono et al. 1995], etc. No such applications have been realized (i.e., reduced to practice) so far, because of technical difficulties expounded above. It is generally understood that for useful off-chip spectroscopic applications, emitted power of more than 0.1 -1 mW is needed without sacrifice in tunability [Konopka et al 1994], and in reality this milestone has not been reached so far.
In the U.S. Pat. No. 3,725,213 to Pierce (1973) a superconductive barrier device and its fabrication technology is disclosed, which, besides other aims, provides for a generator or detector of millimeter and sub-millimeter radiation, based on a granular or particulate structure of the superconductor material. While enhanced radiation emission or sensitivity is intended by the summation of many Josephson junction-containing grains, there is little control and reproducibility, no phase locking, and complete load mismatch to vacuum. Although this device is capable of switching between the superconducting and normal conductivity state by means of a magnetic field, generated by an electrical pulse through a layer adjacent to the Josephson junction, no intention has been made to control the radiation emission frequency by virtue of the effect the magnetic field might have on the energy gap of the superconductor.
A superconducting device is disclosed in U.S. Pat. No. 4,837,604 to Faris (1989), which comprises a plurality of Josephson junctions, stacked vertically on top of one another, with series connection of stacks. It is tailored to a three terminal switch, intended to replace single junctions and lateral arrays of junctions in analog and digital switching applications. Radiation emission is not an aim of that device neither would the chosen design suite such objective.
In U.S. Pat. No. 5,114,912 to Benz (1991), a high-frequency oscillator based on a two-dimensional array of Josephson junctions is described. It is excited by the dc control current from an appropriate current source. The frequency of Josephson oscillations can be tuned continuously by adjusting this dc bias current.
Impedance matching to load can be achieved by selecting the appropriate number of Josephson junctions in the array or by adding resistive shunts. The perceived application of the device is as a tunable dc-to-ac converter at GHz and even THz frequencies.
One drawback of this device is that it is explicitly restricted to two-dimensional planar arrangements of Josephson junctions, which are placed next to one another on the chip. This geometry introduces severe limitations on the maximum possible number of junctions in such an array. Namely, the minimum area of a single junction is around 1 xcexcm2 because of limitations of photolithographic technology, uniformity requirements, the need to have a substantial critical current (not less than 1 mA for optimum power) and low-contact-resistance top lead (or leads). On the other hand, the phase-locking requirement restricts the lateral dimension of the array to about xcex/4, which is about 75 xcexcm for xcexd=1 Thz. Allowing for some spacing between junctions etc., one gets something like 1-2,000 for the maximum number of junctions in a phase-locking array of this design. In practice, arrays of 10xc3x9710 =100 junctions were made. As we will discuss in Section V (vii) below, this design does not allow for out-of-chip power of emitted microwave radiation at a level interesting for conceivable applications i.e. at least 0.1-1 mW. We will argue that alternative designs proposed here (in Section V) allow for much denser packing of Josephson junctions, artificial or intrinsic, and open prospects for sources with much higher emitted power, and by virtue of this property, for a variety of novel applications which are not possible with the planar array devices.
In the E.U. patent EP 446146 to Harada and Hozak (1987) a trilayer Josephson junction is disclosed, comprised of top and bottom superconducting electrodes made of LyBa2Cu3O4, where Ly is Y or a rare-earth element, and 6 less than y7, and a non-superconducting barrier made of Bi2YxSryCuzOw, where 0xe2x89xa6xc3x97xe2x89xa6, 1xe2x89xa6yxe2x89xa63, 1 xe2x89xa6zxe2x89xa63, and 6xe2x89xa6wxe2x89xa613. In this patent, no information was provided about the properties of such junctions (such as Ic, Rn, I-V characteristics, microwave modulation properties, etc.). Nor is there any mention of formation of arrays, vertical or lateral, their expected properties, or applications.
A magnetic control mode for the emission frequency has been proposed for a device disclosed in the European Pat. No. 513,557 to Schroder (1992), where the device of that invention contains stacks of Josephson tunnel junctions, in a series superconducting connection.
In between each pair of neighboring SIS Josephson junctions, there is one more superconducting layer, which is insulated on both sides from the neighboring Josephson junctions . This layer is intended to be used as the control line: by running a current laterally through this layer, as already proposed in U.S. Pat. No. 3,725,213, one should generate a magnetic field which is intended to affect the SIS Josephson junctions by reducing the gap in the superconducting layers.
This device has several drawbacks, some of which make its reduction to practice impossible within the constraints of currently known materials parameters and available microfabrication technologies.
In particular, that patent description does not teach one how to fabricate the (series) superconducting contacts between superconducting electrodes of stacked SIS Josephson junctions, which is the critical technological step required to fabricate the device. It requires one to deposit superconducting pads of dimensions of about 1 xcexcmxc3x970.01 xcexcm, on two opposite lateral sides of a vertical layered column structure.
There is no known technology today capable of performing such a task.
A further concern is the thickness required for the superconducting control line (S2 in FIG. 1 of EP 513.557 A2) in order that it can generate a magnetic field strong enough to reduce the gap in the superconducting electrodes. Take, for example, a layer which is 10 nm thick, within a 1 xcexcm2 mesa.
Such small area mesas are required to keep the critical current of Josephson junction""s small enough for the desired phase-locked operation, as will be expounded later. Asuming a very high critical current density of jc=106A/cm2 in this layer, one gets Ic=a2 jc=106 A/cm2xc3x971 xcexcm xc3x9710 nm=106 A/cm2xc3x9710xe2x88x921 cmxc3x9710xe2x88x926 cm=10xe2x88x924 A. At a distance of about 10 nm, this current would produce a magnetic field of about B≈0.01 Tesla. Such a field is several orders of magnitude too small to significantly affect the critical temperature and the superconductivity gap in the neighbouring superconducting electrodes. In high-Tc superconductor materials, such as YBa2Cu3O7 or Bi2Sr2CaCu2Og, magnetic fields of several Tesla are needed to get a measurable effect on Tc, and that only if the field is oriented perpendicular to the CuO2 planes. In the geometry given in EP 513557 A2, the magnetic field would essentially be parallel to the CuO2 planes.
In this unfavorable geometry, even the highest magnetic fields available today (over 50 T) essentially do not affect Tc in high-Tc superconductor materials, such as Bi2Sr2CaCu2Og. To overcome this difficulty, one would have to use much thicker control-line superconducting layers, say 1 xcexcm thick or even thicker (e.g., 6 xcexcm thick, as in U.S. Pat. No. 3,725,213). That, however, clashes with the current limitations for epitaxial growth of high-quality high-Tc superconductor films (no more than few hundred nm). To fabricate a stack of several such devices would be even less possible.
An alternative approach would be to use some conventional (low-Tc) superconducting materials with much lower critical fields, i.e., much more sensitive to an applied magnetic field. However, apart from losing the advantages of a high Tc (such as the possibility to avoid expensive and cumbersome very low temperature cryogenic systems), in this hypothetical embodiment of the discussed device, one would also lose the advantage of extremely thin superconducting electrodes which are possible if one employs the high-Tc superconductor cuprate compounds. The superconductivity coherence lengths are much larger in conventional low-Tc superconductors, and for that reason one would have to use much thicker superconducting electrodes. This, in turn, will also limit in practice the number of devices in a stack to only, few in contrast to what is assumed in the description of the device in EP 513557 A2.
On the other hand, while it is clearly impractical to modulate the gap and Tc of the superconducting electrodes as proposed in EP 513557 A2, it is possible to introduce vortices in the barrier and control the critical current by an applied magnetic field. We will actually make use of this later in Section V.
Another problem is that of the device xe2x80x9ccross-talkxe2x80x9d. Imagine that one could somehow resolve the problem of control lines and make them of some material that can carry strong enough currents and generate magnetic fields that can reduce the superconducting gap in the neighboring high-Tc superconductor electrodes. The problem is that such a field would affect more than just one Josephson junction. To begin with, according to the design in EP 513 557 A2, for each control line there are two equidistant Josephson junctions, which should be equally affected. But since the magnetic field in the geometry under discussion will fall off slowly as a function of the distance from the control line, one would actually expect that it would affect every Josephson junction within that stack. So, individual Josephson junction control, although the principal aim of that proposal, is impaired with such a design.
A further major-drawback of the device design in EP 513 557 A2 is that no considerations were made of output power of the electromagnetic radiation to be generated. In particular, load matching to vacuum was not considered. As pointed out above, it would be impractical to make even a few-junction stack within that design. This would imply a substantial output-impedance load mismatch. In this case, most of the microwave radiation power would be reflected back and dissipated within the device itself The device would burn out before one is able to extract significant microwave power out of it.
In fact, optimization of a high-Tc superconductor Josephson junction array for maximum output power requires in general both series and parallel superconducting connections, as we will show in detail in the upcoming section V.
To summarize, it is our conclusion that the device described in EP 513557 A2 has not been reduced to practice because of several drawbacks in its design, namely, (i) its fabrication requires the deposition of superconducting contacts about 0.01 xcexcm wide on lateral sides of mesas that contain Josephson junction stacks, for which there is currently no known technology, (ii) there are no known superconductors that can sustain currents large enough to generate magnetic fields strong enough to reduce the superconducting gap in the high-Tc superconductor Josephson junction electrodes, within the dimensional constraints of the device, (iii) the magnetic fields if generated would affect more than one Josephson junction (i.e., there would be inadmissible xe2x80x98cross talkxe2x80x99 between individual devices within the same stack), and (iv) output power would be too low for the proposed system applications, in part because the device design does not allow for Josephson junction array circuit optimization.
It is the purpose of this patent to disclose a device of the invention that overcomes all the drawbacks discussed above.
2. Intrinsic Josephson Junction Stacks in Cuprate Superconductors
In the very first paper on cuprate superconductors in 1986, J. G. Bednorz and K. A. Mxc3xcller expressed an opinion that Laxe2x80x94Baxe2x80x94Cuxe2x80x94O is a quasi-two-dimensional superconductor, in view of its pronounced layered structure. Subsequently, this hypothesis has been confirmed by a variety of experimental findings on various cuprate superconductors (see e.g. Bozovic 1991), the most direct of which was the observation of high-Tc superconductivity in one-unit-cell thick layers of Bi2Sr2CaCu2O8 [Bozovic et al. 1994a]. On the other hand, the critical current along the c-axis (i.e., perpendicular to the CUO2 layers) is much smaller than in a direction parallel to the CUO2 planes, However, it is not zero, i.e., supercurrent can run in the c-axis direction. This clearly implies that the planar, quasi-2D superconducting slabs are weakly coupled by Josephson tunneling
In other words, cuprate superconductors can be viewed as natural (native, intrinsic) stacks of Josephson junctions, spaced at 6-25 xc3x85. A theoretical model for a stack of Josephson coupled superconducting layers was introduced already 25 years ago [Lawrence and Doniach 1971], and studied in much detail since then. The predictions include nonlinear I-V characteristics, microwave-radiation induced I-V (Shapiro) steps, and microwave emission from a sample under dc voltage bias.
Indeed, all these signatures were observed in cuprate superconductors, first in Bi2Sr2CaCu2O8 [Keiner et al. 1992] and subsequently also in (PbyBi1xe2x88x92y)2Sr2CaCu2Og, Tl2Ba2Ca2Cu3O10, etc. [Kleiner and Mxc3xcller 1994, Mxc3xcller 1994, 1996] and by several groups [Regi et al. 1994, Irie et al. 1994, Schmidl et al. 1995, Yasuda et al. 1996, Tanabe et al. 1996, Yurgens et al. 1996, Seidel et al. 1996, Xiao et al. 1996]. Most of these results were obtained on small single crystals, but some work was also done on mesas lithographically defined on single crystals or thin films [Schlenga et al. 1995, Mxc3xcller 1996, Schmidl et al. 1995]. In some cases, phase locking of over 1,000 native junctions was observed [Schlenga et al. 1995]. In general, phase locking was only partial, as evidenced by the appearance of multiple branches in I-V characteristics (in all works published so far). Namely, if the Josephson junctions that belong to a stack are not all identical, i.e., if their critical currents vary from one junction to another, they will not all switch to the normal state at the same point as the bias current is increased. Correspondingly, emission from such an array is expected to be a superposition of coherent (narrow-band) and incoherent (broad-band) radiation, and indeed this was observed [Schienga et al. 1995, Mxc3xcller 1996]. These variations in Josephson junction characteristics are believed to arise from imperfections in the crystal growth and in the lithographic process of defining mesa structures, such as variations in the mesa cross-section area. [We will address this problem in Section V. (iv) below]. The highest frequency of the emitted radiation detected so far was xcexd=95 GHz, due to limited detection capabilities [Mxc3xcller 1996]. The power of detected radiation was minuscule, less than 1 pW, partly due to gross load-impedance mismatch. No practical devices or applications have been reported so far, although some speculations about conceivable future applications for sub-millimeter radiation sources were put forward [Schlenga et al. 1995].
It is therefore an object of the present invention to provide means for avoiding some or all of the above difficulties.
It is another object of this invention to provide a novel generator and/or detector of sub-millimeter electromagnetic radiation, which applies simultaneously a plurality of vertically stacked Josephson junctions connected into a two-dimensional network, so that the generation of microwave power is considerably enhanced.
It is another object of this invention to provide a novel sub-millimeter radiation device with an array impedance that allows an impedance matching to the load, this providing maximum off-chip emission power.
It is another object of this invention to provide a novel sub-millimeter radiation device having a remarkably small emission linewidth (less than one millionth of the radiation frequency xcexd), within its sub-millimeter waveband up to the emission frequency of several Thz.
It is another object of this invention to provide a novel sub-millimeter radiation device with an electronic control mode which allows one to continuously vary the emission frequency and/or to tune the detection frequency, over a broad spectral range.
It is another object of this invention to provide a novel sub-millimeter radiation device, whose emitted microwave intensity can be modulated electronically, including an on/off mode, providing also a fast electronic switch for superconducting electronic circuits.
It is another object of this invention to provide a novel sub-millimeter radiation device of which the main emission direction of the wave field (i.e., the propagation vector of the plane wave) can electronically be rotated within the propagation plane, providing for a sweeping property of the device in both the emission and the detection mode.
It is another object of this invention to provide a novel sub-millimeter radiation device, of which the emitted wave field can electronically be split into two or more parts, and each can be controlled separately, allowing among other features for focusing and defocusing of the combined wave field.
It is another object of this invention to provide a novel sub-millimeter radiation device capable of emitting and/or detecting independently, and even simultaneously, at different fixed (pre-determined) or electronically varied and controlled frequency channels.
It is another object of this invention to provide a novel sub-millimeter radiation device of which the emission and detection mode can be inverted by external electronic means.
It is another object of this invention to provide a novel sub-millimeter radiation device, suitable for its incorporation into superconductor/semiconductor hybrid systems.
These and other objects are achieved according to the present invention by providing a two-dimensional network of linear column-shaped superconducting elements, each of which contains a series array of vertically stacked Josephson junctions, being further this column-shaped superconducting elements grouped in a pre-designed manner under electrical contact plates, carrying additionally said contact plates means for an external electronic control In addition to this the claims 1 to 35.